Six police officers were treated in hospital after diving into a river in a bid to rescue five occupants of a car which had crashed into the water.
One woman inside the car died in the incident in Bristol, and a man and woman have been arrested on suspicion of causing death by dangerous driving.
Police said the six officers dived into the River Avon in a bid to save the silver BMW's occupants after it crashed into the water today.
Two other women were being interviewed as witnesses in what the Fire Service described as a "complex" and "unusual" incident.
The crash happened at Temple Back in the city at 4am today and police are appealing for witnesses who may have been in the area at the time.
Inspector Paul Winship, force incident manager at Avon and Somerset Police, said: "At 4 o'clock this morning a car left the road at St Philip's Bridge at Temple Back in Bristol.
"It went into the River Avon. It's known there were five people in the vehicle. One female was declared dead at the scene.
"At least six police officers dived into the river to effect a rescue. They rescued one male and one female.
"They are both currently helping us with our inquiries.
"Two further females are being interviewed as witnesses.
"We are obviously appealing for witnesses. It's a very busy part of Bristol even at 4 o'clock on a Saturday morning.
"Fire and ambulance were also at the scene. We are not looking for anyone else in connection with the incident.
"The scene is still closed while we are continuing with our inquiries."
Of the police officers who dived into the river, he said: "They have been treated at hospital as a precaution."
Denis McCann, area manager at Avon Fire and Rescue Service, said: "We got the initial call at three minutes past four this morning. The incident was located just around the corner from our Bristol city centre fire station.
"We responded from there with a rescue pump which has enhanced rescue equipment on it and then we had a specialist water rescue team from Bedminster, which is one of the neighbouring city centre stations.
"They have access to a rescue boat, inflatable rafts, inflatable walkways and dry suits. They are specially trained to make entry into moving and still waters.
"There was one person trapped in the vehicle who was removed by Fire and Rescue Service personnel using the rescue boat across to a nearby jetty where they were handed over to the Ambulance Service's Hazardous Area Response (HAR) team."
He added: "It was a complex incident to deal with. It was quite an unusual incident and it required all the agencies working together - fire, police and ambulance services - to resolve it, because there was issues of the casualties, there was issues of accounting for anybody else that had been in the car or in the area.
"So there was a lot of effort by all the agencies working together to bring the incident to a resolution."
Speaking about the person trapped in the car, he added: "She was most definitely removed from the vehicle prior to the vehicle being lifted out of the water."
Floral bouquets tonight marked the scene next to where the young passenger met her death.
The victim, named locally as mother-of-one Namara Whisker, 22, was described as "a bubbly, lively" woman who was "the kindest person you could ever meet".
Ms Whisker was believed to be the front-seat passenger when the car, containing friends, plunged into the river below, just off Passage Street in the centre of Bristol.
School friend Shawnika Sergeant, 22, said the pair had become close while studying at the then-Speedwell Technical College in Bristol.
Miss Sergeant said: "She was so pretty, bubbly and a special girl.
"I could not believe it when people started texting me this morning.
"I have known her for so long, it is hard to take."
Another woman, who arrived to pay tribute at a makeshift shrine next to the river, said she was with Ms Whisker in a Bristol nightclub until minutes before the crash happened.
The woman, who asked not to be named but said she was a distant relative, said: "We saw each other in the club, we gave each other a hug.
"I am so glad I had the chance to do that now, because I realise it was goodbye."
One friend, who said her name was Charlene, said Ms Whisker was a guest at her five-year-old son's Christening.
She added: "Namara has a two-month-old son, I can't believe he is going to grow up without his mummy."
Shattered glass still marked the scene of the incident tonight, with a temporary barrier in place to cover the missing section of iron railing.
One woman, who asked not to be named, said she came to look at the spot where her sister "nearly lost her life". She said her sister was one of the passengers in the car, but had "miraculously" survived.
Ms Whisker's friends said they are planning to come together to pay tribute to the "fashion icon" in the next few days.